<i>int</i> v = map.<b>length</b>									<em>// 长度</em>
<i>Iterable&lt;Object&gt;</i> v = map.<b>keys</b>						<em>// 返回映射的所有键 => (a,b,c)</em>
<i>Iterable&lt;Object&gt;</i> v = map.<b>values</b>						<em>// 返回映射的所有值=> (1,2,3)</em>
<i>bool</i> v = map.<b>isEmpty</b>								<em>// 映射是否为空</em>
<i>bool</i> v = map.<b>isNotEmpty</b>							<em>// 映射是否不为空</em>
<i>Iterable&lt;MapEntry&lt;String, int&gt;&gt;</i> v = map.<b>entries</b> 		<em>// 获取 MapEntry</em>